Series 5 Tailplane stall

[n85ae]

I posted this on the other list, here's a heads up to something that happened
to me.

My plane is a Series 5, with IO-240B and with Trim Assist spring.

I was out of 8500 feet descending through a hole in the clouds, steep
nose down with 11 degrees of flaperons, and trimmed. Speed started to build
so I tugged in 22 degrees at about 80+ mph (something I normally do
not do, but I wanted to see if they'd slow the plane). What happened next
was with absolutely no warning.

Few seconds later I was holding a bit of stick pressure to keep the same
nose down attitude. Then with no warning stick control went away and
the nose dropped violently down, and everything in the plane hit the ceiling
or inside of the windshield. I hit my legs into the bottom of the instrument
panel and slammed up in my straps.

I'm pretty sure it went past vertical, and I grabbed the flaperon handle and
pushed it back out of 22, and got the stick forward a bit. Somehow I ended
in a nose down vertical dive and got it back under control. By the time I
pulled out I'd lost well over a thousand feet.

Pretty sure my flaperons blanked the tail, and it tailplane stalled. But I can't
say for sure. Anyway had it been in the pattern I'd have been dead. Which
very well could happen, as I've often thought to tug in 22 degrees
on a fast descent to try to slow down. But luckily never did do it.

I'll let the list figure out what happen, or the test pilots in the lot. Anyway
my personal opinion is that the variable incidence tail, with heavy nose,
the spring assist and too much flaperons all at the wrong time can put you
tail over nose in a bad way, a lot faster than you can imagine.

Regards,
Jeff Hays

[SkyPirate]

Glad you had the altitude to correct it Jeff,..your analogy is probably right on,..as long as the main wing has plenty lift,..the vorticies follow the trailing edge out to the tip then seperate,..when you decend to the point the wing is making some nasty air especially with flap, that can hit the tail surfaces,..I'm sure at your rapid decent the tail flew down into it ,.. an example if you do a loop and you fly back into the wash the wing made at the bottom when you first pulled back on the stick for the loop ..you hit a good bump ,..even though you didnt do a loop ..your decent was from the sounds of it at the same speed down as forward,.so it's basically the same result except you didnt just fly through it ..your tail was in it,..if that makes sense

[jtpitkin06]

I'm sure this all happened in a flash and it may be difficult to remember. I'll need some more info before making an analysis.

When holding the stick pressure to keep the same nose down attitude, which way? Forward pressure or back pressure?

Next, did you lose the roll control too, or just pitch control?

When you moved the flaperons out of 22 you state "got the stick forward a bit:." Why? Was the nose coming up or was this just instinct to lower the nose when the airplane stalls?

[n85ae]

I've replayed it in my mind a thousand times, and what I did was not that
dramatic, but what happened was. Maybe it was a fluke, and nobody else
will ever have it happen. Anyway, the moment it tossed over like that, I
really had no time to think at all. I don't know how I even reached the
flaperon handle as everything including me was trying to go out the roof.

Regards,
Jeff

[n85ae]

Here's my recollection:

With all that flaperon, even trimmed my plane pitches nose down, so
I was holding some back pressure, When it pitched over the stick
pressure slackened.

It pitched over straight ahead. No bank, or yaw.

I grabbed for the flaperon handle, and all I could manage was to just get
it release from 22 and it snapped forward into 11 degrees position. It was
hard to grab the handle as I was up by the overhead tubing since my seat
belt was not super tight

I pushed forward on the stick out of instinct, but it was so nose over that
I really felt like pulling. This just seemed at the moment the thing to do.
I really did not think about it.

Pretty soon after that the controls stiffened back up, and I started pulling
out around this point I started consciously thinking not to overstress the
plane. However before this, I really didn't think about much

I'm sure this all happened in a flash and it may be difficult to remember. I'll need some more info before making an analysis.

When holding the stick pressure to keep the same nose down attitude, which way? Forward pressure or back pressure?

Next, did you lose the roll control too, or just pitch control?

When you moved the flaperons out of 22 you state "got the stick forward a bit:." Why? Was the nose coming up or was this just instinct to lower the nose when the airplane stalls?

[jrevens]

I had a similiar experience in an entirely different airplane. It occured with a forward CG situation (almost at the limit) test flight, while doing full-flap power-on stalls. The down force created by the elevator to maintain a very high nose angle was evidently considerable, and when disturbed air from the flaps destroyed that force (a classic elevator stall), it stayed that way even after the the pitch-over, which was dramatic & violent - I would describe it as similiar to an "English Bunt" maneuver. It felt like the elevator control had broken. The wing was flying. I was going straight nose down, & if I hadn't retracted the flaps I would have ridden it right into the ground. If I had had electric flaps instead of that good old flap handle staring at me, I'm not sure I would have done the right thing in time. Beware of forward CG, high AOA, and potential air-disturbing devices such as flaps.

I don't know if this is at all helpful on this forum, but Jeff's experience sure brought back memories.

[n85ae]

Hi -

That sounds very similar to what I had happen. Not the kind of thing you
expect. Just one second your flying along, and the next it's "Whoa, what
heck ..."

Where do you fly from? I learned to fly out at KBJC and get out there a
couple times a year.

Regards,
Jeff

[rogerh12]

Is this Flap induced Elevator Stall caused mostly by the inboard section of the flaps?

[jtpitkin06]

Thank you for sharing your incident with us. I hope others can learn from your experience.

I believe you experienced flap overspeed.

All airfoils have a maximum speed limit. The reason for the limit can be structural, where flutter can destroy the airframe in seconds; or, aerodynamic, where controlability becomes an issue. The aircraft might become super sensitive or non-responsive.

Let’s see if we can clear up what probably happened to you.

Extending flaps or flaperons dramatically increases the camber and extends the chord. Both changes move the center of lift aft. The center of lift shifting aft pitches the aircraft forward.

The pilot normally corrects for the forward pitch by trimming nose up. Some aircraft are unable to put in sufficient nose up trim requiring the pilot to hold a small amount of back pressure. Because the aircraft is not in trim it is easy for the speed to increase when the pilot’s attention is diverted.

When a wing produces lift, some air is deflected downward in the form of a wake. The wake or downwash aids the down force normally required by the stab/elevator to maintain pitch.

At higher speeds, with flaps/flaperons extended, pitch control becomes very sensitive. The amount of downwash can change very quickly, up or down, with small inputs from the elevator. The pilot can get into pilot induced oscillations with almost imperceptible inputs.

Elevator stick pressure may become very light due to rapid changes in downwash forces. It is possible for the aircraft to pitch up rapidly when exceeding the maximum flap speed resulting in excessive G forces. The aircraft may also pitch down rapidly leading a pilot to think the elevator has stalled. A rapid pitch down lowers the amount of downwash on the elevator. The pitch control may suddenly feel “mushy” even though it is still functional.

This phenomenon is not a blanking of the tail or a stalling of the stab. It is a case of flap overspeed where pitch control becomes extremely sensitive. It is not something peculiar to the Kitfox. It can happen in any aircraft equipped with flaps or flaperons.

To recover from flap overspeed the pilot must slow the aircraft or reduce the flap extension. Immediate speed reduction with steep negative pitch is almost impossible, so flap reduction is the action of choice.

You were very sharp to dump the flaps. Doing so put the aircraft back into a controllable state with partial flaps. Atta Boy!!!

The rapid pitch down slammed you upward with negative G’s. The sensation was the same as you might get from an accelerated stall. Your instinctive reaction from all your previous flight training was to lower the nose. I can’t fault you for that as you were in your own corner of uncharted territory.

I suspect your aircraft accelerated beyond the maximum flap speed with just a moment of inattention while you were looking at your descent path. It can occur very easily when near the maximum if you are out of trim. If your target speed was 80 you could easily pop up to 90, 100 or more in a blink.
We tend to think of the red line as the only maximum speed limit. We see that the maximum speed for flaps at the top of the white arc is just as important as the ultimate red line.

I ‘m pretty confident a flap overspeed is what happened to you. We’re happy you are here to share your story.

Regards,
John Pitkin

[n85ae]

That makes sense. I initially thought I'd done some kind of high speed stall,
then I started thinking tailplane stall. I'm still not sure, it was not particularly
pitch sensitive. I'd say it felt positive in control until the moment there was
no control.

I was very aware of the flaperons as I had dumped them in intentionally, so
it wasn't requiring much thought when it happened that they had caused it.

John at Kitfox said he was going to see if he could reproduce it based on my
description, if he does I'm sure he'll let me know. If so I'll post what he finds.

Regards, Jeff Hays